APS are solid state imagers where each pixel contains a photo-sensing means, charge to voltage conversion means, reset means, and all or part of an amplifier. They have some advantages over charge coupled device imagers (CCD), including single 5V supply operation, x-y addressability, and on-chip signal processing.
APS are solid state imagers where each pixel contains a photo-sensing means, charge to voltage conversion means, reset means and all or part of an amplifier. Because of the approach used in resetting the pixel, APS devices have been operated in a manner where each line or row of the imager is integrated, reset, and read out at a different time interval than each of the remaining lines or rows. Hence if one were reading out the entire imager, each line would have captured the scene at a different point in time. Since illumination conditions can and do vary temporally, and since objects in the scene may also be moving, this method of read out can produce line artifacts in the resulting representation of the imager. This limits the usefulness of APS devices in applications where high quality motion or still images are required.
Additionally, this method of operation (reset and read a row at a time), requires that each column of the device have a separate clamp and sample amplifier. It is often the physical size of this amplifier as realized in the process used to fabricate the device that limits the pixel size of the sensor. In order to build high resolution, small pixel APS devices, it is necessary to use sub-.mu.m CMOS processes to realize the same resolution and sensitivity APS device when compared to a standard charge coupled device (CCD) sensor. The minimum pixel size of an APS device is typically 15.times. to 20.times. of the minimum feature size of the process technology used, compared to 5.times. to 10.times. for a CCD device.
From the foregoing discussion, it should be apparent that there remains a need within the art of APS devices for the provision of a reset mechanism that provides the capability to reset all pixels simultaneously as well as for selectively providing reset of a pixel at a time. Additionally, there remains a need in the art for a circuit design that will provide simultaneous and equal integration times of each pixel and enable use of a single clamp and sample amplifier per device rather than per column.